It is desirable that thermoplastic resin powder has high bulk specific gravity in view of efficient utilization of the storage space of the powder as a product and incidental reduction of its transportation cost.
Hitherto, many studies have been made on the method for elevating bulk specific gravity of thermoplastic resin powder. For example, many researches have been conducted on means for improving powder properties of the powdery or granular polymers recovered from graft copolymer latices containing an elastomeric backbone polymer in a high ratio. For instance, U.S. Pat. No. 4,897,462 discloses a method for carrying out slow coagulation of a polymer latex in the presence of a coagulant of a specific concentration. Also, U.S. Pat. No. 4,275,178 discloses a method for improving the powder properties such as bulk specific gravity and blocking resistance of the produced coagulated slurries or dry powders by adding a hard polymer thereto.
The former method characterized by the slow coagulation in the presence of a coagulant of a specific concentration is capable of providing a spherical powder with a narrow particle size distribution, but it has disadvantages in that because of high water content of the powder after dehydration, when a heat treatment is carried out at a high temperature during solidification for obtaining the solid particles, the fine individual particles in the coagulated particles may be fused together without compacted sufficiently, making it unable to obtain the coagulated particles with high density and, consequently there can not be obtained the particles of high bulk specific gravity.
The latter method characterized by the addition of a hard polymer for improving the powder properties is designed to elevate bulk specific gravity of the powder by densifying the filled structure of the coagulated particle agglomerates by such means as cohesion of the coagulated particles, but this method has the problem that the effect of improving the density of the coagulated particles themselves can not be expected and a powder with a well satisfactory bulk specific gravity can not be obtained.
The present inventors have pursued further studies with a view to solving the above problems and, as a result, found that a polymer powder with a high bulk specific gravity comprising the compacted particles can be obtained by adding a specific polymer and/or an inorganic compound to a coagulation slurry or polymer powder obtained from a thermoplastic polymer latex and compressing and comminuting the resulting polymer powder. The present invention has been attained on the basis of this finding.